1. Field of the Invention
This invention relates to devices for storing fluids, whether liquid-gaseous or completely gaseous, under high pressure and to methods for making such devices.
2. Background Art
Hydrogen can be stored in a solid state as a chemical hydride or a metal hydride, cryogenically as a liquid or as a liquid-gaseous hybrid with refrigeration and/or insulation accompanied by gradual boil-off, or as a gas under very high pressure.
Other gases, such as but not limited to, oxygen, nitrogen, and carbon dioxide, can be stored in similar manner to hydrogen, i.e., as a cryogenic liquid, possibly as a cryogenic liquid-gas combination, or as a pressurized gas. Hydrogen and oxygen illustrate the need for and challenges of portable gas storage.
A major application of portable hydrogen storage is for use in hydrogen-based vehicles, where enough hydrogen is needed for an acceptable driving range. That range must be achieved with a storage tank that neither consumes inordinate space, nor adds excessive empty-tank weight to the vehicle.
A common application of portable oxygen storage is for home/personal oxygen therapy in the medical field. Storage of oxygen sufficient for a day away from a non-portable system requires a tank that is often unmanageable without the use of a wheeled cart, or sometimes via a carried shoulder sling. While conservor systems, which impede oxygen flow during exhalation, make these systems more efficient, 10 hours of oxygen at a low prescription of 2 lpm requires a ‘D’ tank. Such tanks are typically 100 mm in diameter, 400 mm tall, and when empty weigh over 5 lbs. Ambulatory (active) patients may consume oxygen at 2-3 times that rate.
Another application of portable oxygen storage tanks is in fire and rescue self-contained breathing apparatus (SCBA's). Similar to that is storage of compressed air for diving (SCUBA) applications. In SCBA applications, one wishes to minimize weight. In SCUBA applications, weight is not as much an issue, as neutral buoyancy is attractive.
All these applications utilize tanks having a shape that is commonly cylindrical. Such a shape provides the most efficient containment of pressure and volume for a given strength of material that makes up the outer shell of the hollow container.
Recent advancements have led to reducing the mass of tanks for SCBA, in the form of carbon-fiber tanks. For healthcare applications, aluminum tanks are now used. The most recent advancements in high-pressure portable storage have been motivated by the need for vehicular hydrogen storage.
One commercial embodiment of high-pressure hydrogen storage is Quantum's 10 ksi (23.5 ksi burst pressure) tank that extends a previous 5 ksi tank (Quantum Fuel Systems Technologies Worldwide, Inc., Irvine, Calif.). These tanks are believed to utilize an inner liner made of a high molecular weight polymer that serves as a hydrogen gas permeation barrier. The barrier is then surrounded by a carbon fiber-epoxy resin composite shell that is the pressure load-bearing component of the tank. Preferably, such systems should be resistant to hydrogen corrosion and deploy materials that are not affected by stress corrosion cracking and hydrogen embrittlement.
As noted, hydrogen can also be stored in a solid state through absorption and recovery of hydrogen from the surface of the substrate where for example, a chemical hydride forms. These storage approaches suffer from high tank mass, and aim to improve storage efficiency by maximizing the surface area to weight ratio of the material in which the storage occurs. For example, foams with very small cell/void sizes, even aerogels (“solid smoke”), provide good surface area to weight ratios, but are still heavy in relation to the mass of hydrogen that can be absorbed.